1. Field of the Invention
This invention relates to an electron gun for a cathode ray tube used in a projector tube, a color TV tube and an index tube and the like, for example.
2. Description of Related Art
As the related art electron gun for a cathode ray tube, the gun shown in FIG. 33, for example, is well known in the art.
This electron gun is of a uni-potential type, wherein the first to fifth grids acting as accelerator electrodes and focusing electrodes are coaxially (a Z-axis) arranged against a cathode K for discharging electrons. Then, electron beams discharged from the cathode K are focused on a fluorescent surface under an action of pre-focusing lens formed by the second and third grids G.sub.2, G.sub.3 and an action of a main lens formed by the third to fifth grids G.sub.3 to G.sub.5. These cathode K and the first to fifth grids G.sub.1 to G.sub.5 are fixed to a beading glass through melting and integrally assembled. In addition, the first to fifth grids G.sub.1 to G.sub.5 are made of metal such as stainless steel, for example.
However, such electron gun had some problems as described below.
That is, in the aforesaid configuration, a certain displacement may easily occur in a degree of concentricity of electrodes, the third to the fifth grids G.sub.3 to G.sub.5, resulting in that the electron beams may be moved away from an axis to cause a blooming of the electron beams to be easily generated.
In addition, since there was a high potential gradient between the electrodes, an electrical discharging was apt to occur among the third to the fifth grids G.sub.3 to G.sub.5, a spherical aberration of a lens diameter was increased to cause a beam spot diameter to be increased.
In addition, when a gap between the third grid G.sub.3 and the fourth grid G.sub.4 and another gap between the fourth grid G.sub.4 and the fifth grid G.sub.5 are widened by more than a certain space, this operation shows a problem that the electron beams are leaked out and a charge-up occurs at the neck part or the beading glass. These gaps are related to a performance of an electron gun (in particular, a coefficient of spherical aberration) and so it is desired to make the most suitable gap.